The mark of the Prophet (saas) is on his shoulder.(Al-Barzanji, Al-Isha'ah li-ashrat al-sa'ah, p. 163)

Our Prophet (saas) has revealed that there will be A SIGN BELONGING TO OUR PROPHET (SAAS) on Hazrat Mahdi’s body. Other hadiths state that THIS SIGN IS THE PROPHET’S SEAL OF PROPHETHOOD and this is compared to the PROPHET’S MARK. This is yet another astonishing detail about Hazrat Mahdi (as).
As we know, THE SEAL IS DARK IN COLOR and COVERS THE SURFACE.
In addition, the location of this mark on Hazrat Mahdi’s (as) back is pinned down by the provision of specific coordinates. Accounts state that every prophet has the seal of prophethood on his right hand. The seal of our Prophet (saas) is described as being “at the level of his heart on the skin of his left shoulder blade.” The reference in the hadiths to Hazrat Mahdi (as) having the seal of prophethood of our Prophet (saas) on his shoulder shows that he will also have “A DARK MARK AT THE LEVEL OF HIS HEART ON HIS BACK, ON HIS LEFT SHOULDER BLADE.”

Ibrahim bin Muhammad, one of the grandsons of Hazrat Ali, relates: “In describing the characteristics of our Prophet (saas), my grandfather Hazrat Ali would relate all the hadith about the physical appearance of the Messenger of Allah (saas) at length and say:

‘There was the seal of prophethood between his shoulder blades. And he is the last of the prophets.’” (At-Tirmidhi, Imam Abu `Isa Muhammed ibn `Isa Tirmidhi, Shamail Sharif, Vol 1. Hilal Press, Ankara, 1976, p. 38)